Fresh evidence has emerged of the extent of destruction and appalling conditions in Falluja, still deserted two months after a major US offensive against the insurgent stronghold.

Ali Fadhil, an Iraqi journalist working with the Guardian's film unit and one of the few reporters to travel independently to Falluja, describes in a Channel 4 News film tonight a "city of ghosts" where dogs feed on uncollected corpses.

In interviews, insurgents challenge official US accounts of a decisive victory and claim many of the rebels left the city in a pre-planned withdrawal.

"It is completely devastated," Fadhil writes in the Guardian today. "Falluja used to be a modern city; now there is nothing. We spend that first day going through the rubble that had been the centre of the city; I don't see a single building that is functioning."

Most of Falluja's 300,000 residents fled before the assault and now some have begun to return to find their homes destroyed, the water and electricity still cut and untreated sewage flowing openly. There is little chance elections can be held there with polling day three weeks away.

Some Iraqis openly criticise the fighters, despite the risks. "The mujahideen are responsible and the clerics for the destruction that happened to our city; no one will forgive them for that," a former major in the much feared Republican Guard tells Fadhil.

In one badly damaged home near a cemetery, he finds the body of a fighter still lying on the floor. "The leg is missing, the hand is missing and the furniture in the house has been destroyed," he writes. "I can't breathe with the smell."

US commanders claimed to have killed more than 1,200 insurgents in the November battle, dealing a serious blow to the insurgency. Before the assault, Falluja was a no-go area for the US and Iraqi military.

But in a graveyard, known as the "martyrs cemetery", Fadhil counts only 76 graves. In houses he finds other bodies he suspects were civilians.

"I saw other rotting bodies that showed no sign of being fighters. In one house in the market there were four bodies inside the guestroom," he writes. "In this house there were no bullets in the walls, just four dead men lying curled up beside each other, with bullet holes in the mosquito nets that covered the windows."

The allegations were put to US forces in Baghdad five days ago. There has been no reply.

Despite the intense fight in Falluja, the insurgency has gathered pace across Iraq, particularly in the northern city of Mosul, once a model of peace and calm, and in Baghdad, where the deputy police chief was assassinated yesterday.

US commanders thought the rebels had been surrounded in Falluja. Yet one fighter tells Fadhil his men left 10 days into the battle: "We did not pull out because we did not want to fight. We needed to regroup; it was a tactical move."